The carboxy terminus of human immunodeficiency virus type 1 gp160 limits its proteolytic processing and transport in transfected cell lines

In-solution virus capture assay helps deconstruct heterogeneous antibody recognition of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) on whole virions is heterogeneous, so molecular analysis of Env with monoclonal antibodies (MAbs) is challenging. Virus capture assays (VCAs) involving immobilized MAbs are typically used, but these assays suffer from immobilization artifacts and do not provide binding constants. Furthermore, we show here that certain HIV-1 neutralizing MAbs, including 2G12, 4E10, 2F5, Z13e1, and D5, will capture virion particles completely devoid of Env. We modified the VCA such that MAbs and virions are incubated in solution, and unbound MAbs are removed prior to the capture step. This modification nearly eliminated evidence of Env-independent binding by MAbs to virions and allowed determination of apparent affinity constants in solution. Three important qualitative observations were further revealed. First, neutralizing MAbs 2F5, 4E10, and Z13e1 against the membrane-proximal external region (MPER) of HIV-1 gp41 were found to capture virions efficiently only if a significant amount of uncleaved gp160 or synthetic MPER peptide was present. Second, we show how non-native forms of Env vary by Env genotype and that Env from HIV-1(JR-FL) is more homogeneously trimeric than that from HIV-1(JR-CSF). Third, we determined that Env containing all or parts of gp41, including uncleaved gp160, binds spontaneously to free virions. This exogenous Env is an indiscriminate molecular "bridge" between Env-specific Ab and virions and can affect VCA analyses, particularly using pseudotyped virions. Heterogeneity in Env from endogenous and exogenous sources might also subvert humoral immunity to HIV-1, so in-solution VCAs may help to dissect this heterogeneity for vaccine design purposes.

Enhanced cell surface expression, immunogenicity and genetic stability resulting from a spontaneous truncation of HIV Env expressed by a recombinant MVA

During propagation of modified vaccinia virus Ankara (MVA) encoding HIV 89.6 Env, a few viral foci stained very prominently. Virus cloned from such foci replicated to higher titers than the parent and displayed enhanced genetic stability on passage. Sequence analysis showed a single nucleotide deletion in the 89.6 env gene of the mutant that caused a frame shift and truncation of 115 amino acids from the cytoplasmic domain. The truncated Env was more highly expressed on the cell surface, induced higher antibody responses than the full-length Env, reacted with HIV neutralizing monoclonal antibodies and mediated CD4/co-receptor-dependent fusion. Intramuscular (i.m.), intradermal (i.d.) needleless, and intrarectal (i.r.) catheter inoculations gave comparable serum IgG responses. However, intraoral (i.o.) needleless injector route gave the highest IgA in lung washings and i.r. gave the highest IgA and IgG responses in fecal extracts. Induction of CTL responses in the spleens of individual mice as assayed by intracellular cytokine staining was similar with both the full-length and truncated Env constructs. Induction of acute and memory CTL in the spleens of mice immunized with the truncated Env construct by i.d., i.o., and i.r. routes was comparable and higher than by the i.m. route, but only the i.r. route induced CTL in the gut-associated lymphoid tissue. Thus, truncation of Env enhanced genetic stability as well as serum and mucosal antibody responses, suggesting the desirability of a similar modification in MVA-based candidate HIV vaccines.

Immunogenicity testing of a novel engineered HIV-1 envelope gp140 DNA vaccine construct

DNA vaccines expressing the envelope (env) of the human immunodeficiency virus type 1 (HIV-1) have been relatively ineffective at generating strong immune responses. In this study, we described the development of a recombinant plasmid DNA (pEK2P-B) expressing an engineered codon-optimized envelope gp140 gene of primary (nonrecombinant) HIV-1 subtype B isolate 6101. Codon usage and RNA optimization of HIV-1 structural genes has been shown to increase protein expression in vitro as well as in the context of DNA vaccines in vivo. To further increase the expression, a synthetic IgE leader with kozak sequences were fused into the env gene. The cytoplasmic tail of the gene was also truncated to prevent recycling. The expression of env by the recombinant pEK2P-B was evaluated using T7 coupled transcription/translation. The construct demonstrated high expression of the HIV-1 env gene in eukaryotic cells as demonstrated in transfected 293-T and RD cells. Immunogenicity of pEK2P-B was evaluated in mice using IFN-gamma ELISpot assay, and the construct was found to be highly immunogenic and crossreactive with HIV-1 clade C env peptides. Three immunodominant peptides were also mapped out. Furthermore, by performing a CFSE flow cytometry-based proliferation assay, 2.4 and 1.5% proliferation was observed in CD4+, CD8+, and CCR+ memory T cells, respectively. Therefore, this engineered synthetic optimized env DNA vaccine may be useful in DNA vaccine and other studies of HIV-1 immunogenicity.

Virion envelope content, infectivity, and neutralization sensitivity of simian immunodeficiency virus

A truncating E767stop mutation was introduced into the envelope glycoprotein of simian immunodeficiency virus (SIV) strain SIV239-M5 (moderately sensitive to antibody-mediated neutralization and lacking five sites for N-linked carbohydrate attachment) and strain SIV316 (very sensitive to neutralization, with eight amino acid changes from the neutralization-resistant parental molecular clone, SIV239). The truncating mutation increased Env content in virions, increased infectivity, and decreased sensitivity to antibody-mediated neutralization in both strains. However, the magnitude of the effect on infectivity and neutralization sensitivity differed considerably between the two strains. In the context of strain SIV239-M5, truncation increased Env content in virions approximately 10-fold and infectivity in a reporter cell assay 24-fold. The truncated SIV239-M5 was only slightly more resistant to neutralization by polyclonal monkey sera and by monoclonal antibodies than SIV239-M5 with a full-length envelope glycoprotein. In the context of strain SIV316, truncation increased infectivity a dramatic 480-fold, while envelope content in virions was increased only about 14-fold. This dramatic increase in infectivity cannot be simply explained by the increase in envelope content and is likely due to an increase in inherent infectivity, i.e., infectivity per spike, that results from truncation. The truncated SIV316 was extremely resistant to antibody-mediated neutralization. In fact, it was not neutralized by any of the antibodies tested. When increasing amounts of SIV316 envelope glycoprotein (full length) were provided in trans to SIV316, infectivity was increased and sensitivity to neutralization was decreased, but to nowhere near the degree that was obtained when truncated SIV316 envelope glycoprotein was used. Truncated forms of SIV239 and SIV239-M5 required higher levels of soluble CD4 for inhibition of infection than their nontruncated forms; truncated SIV316 did not. Our results suggest that envelope content in SIV virions, infectivity, and resistance to antibody-mediated neutralization can be increased not only by truncation of the cytoplasmic domain but also by provision of excess envelope in trans. The striking increase in infectivity that results from truncation in the context of SIV316 appears to be due principally to an increase in inherent infectivity per spike.

Surface stability and immunogenicity of the human immunodeficiency virus envelope glycoprotein: role of the cytoplasmic domain

The effects of two functional domains, the membrane-proximal YXXPhi motif and the membrane-distal inhibitory sequence in the long cytoplasmic tail of the human immunodeficiency virus type 1 (HIV-1) envelope protein (Env), on immunogenicity of the envelope protein were investigated. Genes with codons optimized for mammalian expression were synthesized for the HIV 89.6 Env and a truncated Env with 50 amino acids in the cytoplasmic domain to delete the membrane distal inhibitory sequence for surface expression. Additional genes were generated in which the tyrosine residue in the YXXPhi motif was changed into a serine. Pulse-chase radioactive labeling and immunoprecipitation studies indicated that both domains can mediate endocytosis of the HIV Env, and removal of both domains is required to enhance HIV Env protein surface stability. Analysis of immune responses induced by DNA immunization of mice showed that the DNA construct for the mutant Env exhibiting enhanced surface stability induced significantly higher levels of antibody responses against the HIV Env protein. Our results suggest that the HIV Env cytoplasmic domain may play important roles in virus infection and pathogenesis by modulating its immunogenicity.

Enhancement of immunogenicity of an HIV Env DNA vaccine by mutation of the Tyr-based endocytosis motif in the cytoplasmic domain

We investigated the effect of the conserved tyrosine-based endocytosis motif (YXXPhi) in the cytoplasmic domain of the human immunodeficiency viruses (HIV) envelope protein (Env) on its immunogenicity. Genes with codons optimized for mammalian expression were synthesized for the HIV 89.6 Env with a truncated cytoplasmic domain and a mutant Env in which the tyrosine residue in the YXXPhi motif was changed into a serine. Mutation of the Tyr residue enhanced surface expression of the Env protein. Analysis of immune responses induced by DNA immunization of mice showed that the DNA construct for the Tyr mutant Env induced moderately higher levels of T cell responses. More interestingly, the DNA construct for the mutant Env induced significantly higher levels of antibody responses against the Env protein in comparison to the construct for the wild type Env. Our results suggest that the YXXPhi motif in the HIV Env cytoplasmic domain may play a role in virus evasion of host immune responses through affecting its immunogenicity.

Cellular membrane-binding ability of the C-terminal cytoplasmic domain of human immunodeficiency virus type 1 envelope transmembrane protein gp41

The amphipathic alpha-helices located in the cytoplasmic tail of the envelope (Env) transmembrane glycoprotein gp41 of human immunodeficiency virus type 1 have been implicated in membrane association and cytopathicity. Deletion of the last 12 amino acids in the C terminus of this domain severely impairs infectivity. However, the nature of the involvement of the cytoplasmic tail in Env-membrane interactions in cells and the molecular basis for the defect in infectivity of this mutant virus are still poorly understood. In this study we examined the interaction of the cytoplasmic tail with membranes in living mammalian cells by expressing a recombinant cytoplasmic tail fragment and an Escherichia coli beta-galactosidase/cytoplasmic tail fusion protein, both of them lacking gp120, the gp41 ectodomain, and the transmembrane region. We found through cell fractionation, in vivo membrane flotation, and confocal immunofluorescence studies that the cytoplasmic tail contained determinants to be routed to a perinuclear membrane region in cells. Further mapping showed that each of the three lentivirus lytic peptide (LLP-1, LLP-2, and LLP-3) sequences conferred this cellular membrane-targeting ability. Deletion of the last 12 amino acids from the C terminus abolished the ability of the LLP-1 motif to bind to membranes. High salt extraction, in vitro transcription and translation, and posttranslational membrane binding analyses indicated that the beta-galactosidase/LLP fusion proteins were inserted into membranes via the LLP sequences. Subcellular fractionation and confocal microscopy studies revealed that each of the LLP motifs, acting in a position-independent manner, targeted non-endoplasmic reticulum (ER)-associated beta-galactosidase and enhanced green fluorescence protein to the ER. Our study provides a basis for the involvement of the gp41 cytoplasmic tail during Env maturation and also supports the notion that the membrane apposition of the C-terminal cytoplasmic tail plays a crucial role in virus-host interaction.

Identification of two sequences in the cytoplasmic tail of the human immunodeficiency virus type 1 envelope glycoprotein that inhibit cell surface expression

During synthesis and export of protein, the majority of the human immunodeficiency virus type 1 (HIV-1) Env glycoprotein gp160 is retained in the endoplasmic reticulum (ER) and subsequently ubiquitinated and degraded by proteasomes. Only a small fraction of gp160 appears to be correctly folded and processed and is transported to the cell surface, which makes it difficult to identify negative sequence elements regulating steady-state surface expression of Env at the post-ER level. Moreover, poorly localized mRNA retention sequences inhibiting the nucleocytoplasmic transport of viral transcripts interfere with the identification of these sequence elements. Using two heterologous systems with CD4 or immunoglobulin extracellular/transmembrane domains in combination with the gp160 cytoplasmic domain, we were able to identify two membrane-distal, neighboring motifs, is1 (amino acids 750 to 763) and is2 (amino acids 764 to 785), which inhibited surface expression and induced Golgi localization of the chimeric proteins. To prove that these two elements act similarly in the homologous context of the Env glycoprotein, we generated a synthetic gp160 gene with synonymous codons, the transcripts of which are not retained within the nucleus. In accordance with the results in heterologous systems, an internal deletion of both elements considerably increased surface expression of gp160.

Human immunodeficiency virus type 2 glycoprotein enhancement of particle budding: role of the cytoplasmic domain

Previous studies have shown that the glycoprotein cytoplasmic domains of human immunodeficiency virus type 2 (HIV-2) or simian immunodeficiency virus of macaques modulate biological activities of the viral glycoprotein complex, including syncytium formation, exterior glycoprotein conformation, and glycoprotein incorporation into budding virus particles. We have now utilized a recombinant expression system to study interactions of full-length or truncated HIV-2 glycoproteins with coexpressed HIV-2 Gag proteins which self-assemble and bud as virus-like particles. Interestingly, budding of HIV-2 virus-like particles from cells was enhanced 5- to 24-fold when Gag was coexpressed with the full-length HIV-2 glycoprotein, compared with Gag expressed either alone or with a truncated HIV-2 glycoprotein. The results obtained in this model system indicate that an additional effect of the lengthy cytoplasmic domain of the glycoprotein of HIV-2 is enhancement of particle budding. We speculate that the cytoplasmic domain of the viral glycoprotein of HIV-2 enhances budding by (i) potentiation of Gag structure or function or (ii) membrane modulation.

An internalization signal in the simian immunodeficiency virus transmembrane protein cytoplasmic domain modulates expression of envelope glycoproteins on the cell surface

A Tyr to Cys mutation at amino acid position 723 in the cytoplasmic domain of the simian immunodeficiency virus (SIV) transmembrane (TM) molecule has been shown to increase expression of envelope glycoproteins on the surface of infected cells. Here we show that Tyr-723 contributes to a sorting signal that directs the rapid endocytosis of viral glycoproteins from the plasma membrane via coated pits. On cells infected by SIVs with a Tyr at position 723, envelope glycoproteins were transiently expressed on the cell surface and then rapidly endocytosed. Similar findings were noted for envelope molecules expressed in the absence of other viral proteins. Immunoelectron microscopy demonstrated that these molecules were localized in patches on the cell surface and were frequently associated with coated pits. In contrast, envelope glycoproteins containing a Y723C mutation were diffusely distributed over the entire plasma membrane. To determine if an internalization signal was present in the SIV TM, chimeric molecules were constructed that contained the CD4 external and membrane spanning domains and a SIV TM cytoplasmic tail with a Tyr or other amino acids at SIV position 723. In Hela cells stably expressing these molecules, chimeras with a Tyr-723 were rapidly endocytosed, while chimeras containing other amino acids at position 723, including a Phe, were internalized at rates only slightly faster than a CD4 molecule that lacked a cytoplasmic domain. In addition, the biological effects of the internalization signal were evaluated in infectious viruses. A mutation that disrupted the signal and as a result, increased the level of viral envelope glycoprotein on infected cells, was associated with accelerated infection kinetics and increased cell fusion during viral replication. These results demonstrate that a Tyr-dependent motif in the SIV TM cytoplasmic domain can function as an internalization signal that can modulate expression of the viral envelope molecules on the cell surface and affect the biological properties of infectious viruses. The conservation of an analogous Tyr in all human and simian immunodeficiency viruses suggests that this signal may be present in other primate lentiviruses and could be important in the pathogenesis of these viruses in vivo.

Attachment of an oligopeptide epitope to the C-terminus of recombinant SIV gp160 facilitates the construction of SMAA complexes while preserving CD4 binding

A small 14 amino acid oligopeptide tag (termed SV5-Pk) was fused onto the carboxy-terminus of simian immunodeficiency virus gp160 expressed from a recombinant baculovirus. The presence of the Pk tag had no obvious effect on the expression and glycosylation of gp160 and did not interfere either with CD4 binding or with cleavage at its maturation site by the protease furin. The presence of the Pk tag did, however, facilitate the simplified purification of full-length gp160 and its incorporation into immunogenic solid matrix-antibody-antigen (SMAA) complexes.

Temperature dependence of cell-cell fusion induced by the envelope glycoprotein of human immunodeficiency virus type 1

We investigated cell-cell fusion induced by the envelope glycoprotein of human immunodeficiency virus type 1 strain IIIB expressed on the surface of CHO cells. These cells formed syncytia when incubated together with CD4-positive human lymphoblastoid SupT1 cells or HeLa-CD4 cells but not when incubated with CD4-negative cell lines. A new assay for binding and fusion was developed by using fluorescent phospholipid analogs that were produced in SupT1 cells by metabolic incorporation of BODIPY-labeled fatty acids. Fusion occurred as early as 10 min after mixing of labeled SupT1 cells with unlabeled CHO-gp160 cells at 37 degrees C. When both the fluorescence assay and formation of syncytia were used, fusion of SupT1 and HeLa-CD4 cells with CHO-gp160 cells was observed only at temperatures above 25 degrees C, confirming recent observations (Y.-K. Fu, T.K. Hart, Z.L. Jonak, and P.J. Bugelski, J. Virol. 67:3818-3825, 1993). This temperature dependence was not observed with influenza virus-induced cell-cell fusion, which was quantitatively similar at both 20 and 37 degrees C, indicating that cell-cell fusion in general is not temperature dependent in this range. gp120-CD4-specific cell-cell binding was found over the entire 0 to 37 degrees C range but increased markedly above 25 degrees C. The enhanced binding and fusion were reduced by cytochalasins B and D. Binding of soluble gp120 to CD4-expressing cells was equivalent at 37 and 16 degrees C. Together, these data indicate that during gp120-gp41-induced syncytium formation, initial cell-cell binding is followed by a cytoskeleton-dependent increase in the number of gp120-CD4 complexes, leading to an increase in the avidity of cell-cell binding. The increased number of gp120-CD4 complexes is required for fusion, which suggests that the formation of a fusion complex consisting of multiple CD4 and gp120-gp41 molecules is a step in the fusion mechanism.

HIV-1 proteins in infected cells determine the presentation of viral peptides by HLA class I and class II molecules and the nature of the cellular and humoral antiviral immune responses--a review

The goals of molecular virology and immunology during the second half of the 20th century have been to provide the conceptual approaches and the tools for the development of safe and efficient virus vaccines for the human population. The success of the vaccination approach to prevent virus epidemics was attributed to the ability of inactivated and live virus vaccines to induce a humoral immune response and to produce antiviral neutralizing antibodies in the vaccinees. The successful development of antiviral vaccines and their application to most of the human population led to a marked decrease in virus epidemics around the globe. Despite this remarkable achievement, the developing epidemics of HIV-caused AIDS (accompanied by activation of latent herpesviruses in AIDS patients), epidemics of Dengue fever, and infections with respiratory syncytial virus may indicate that conventional approaches to the development of virus vaccines that induce antiviral humoral responses may not suffice. This may indicate that virus vaccines that induce a cellular immune response, leading to the destruction of virus-infected cells by CD8+ cytotoxic T cells (CTLs), may be needed. Antiviral CD8+ CTLs are induced by viral peptides presented within the peptide binding grooves of HLA class I molecules present on the surface of infected cells. Studies in the last decade provided an insight into the presentation of viral peptides by HLA class I molecules to CD8+ T cells. These studies are here reviewed, together with a review of the molecular events of virus replication, to obtain an overview of how viral peptides associate with the HLA class I molecules. A similar review is provided on the molecular pathway by which viral proteins, used as subunit vaccines or inactivated virus particles, are taken up by endosomes in the endosome pathway and are processed by proteolytic enzymes into peptides that interact with HLA class II molecules during their transport to the plasma membrane of antigen-presenting cells. Such peptides are identified by T-cell receptors present on the plasma membrane of CD4+ T helper cells. The need to develop viral synthetic peptides that will have the correct amino acid motifs for binding to HLA class I A, B, and C haplotypes is reviewed. The development of HIV vaccines that will stimulate, in an uninfected individual, the humoral (antibody) and cellular (CTL) immune defenses against HIV and HIV-infected cells, respectively, and may lead to protection from primary HIV infection are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of stable Chinese hamster ovary cells expressing wild-type, secreted, and glycosylphosphatidylinositol-anchored human immunodeficiency virus type 1 envelope glycoprotein

We generated Chinese hamster ovary cell lines that stably express wild-type, secreted, and glycosylphosphatidylinositol (GPI)-anchored envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1). The cells expressing wild-type Env (WT cells) express both the precursor gp160 and the mature gp120/gp41 and readily form large syncytia when cocultivated with CD4+ human cells. The cells expressing secreted Env (SEC cells) release 140-kDa precursor and mature 120-kDa envelope glycoproteins into the supernatants. The cells expressing GPI-anchored Env (PI cells) express both 140-kDa precursor and mature gp120/gp41 envelope glycoproteins, which can be released from the cell surface by treatment with phosphatidylinositol-specific phospholipase C (PI-PLC). Both the secreted and PI-PLC-released envelope glycoproteins form oligomers that can be detected on nonreducing sodium dodecyl sulfate-polyacrylamide gels. In contrast to the WT cells, the SEC and PI cells do not form syncytia when cocultivated with CD4+ human cells. The availability of cells producing water-soluble oligomers of HIV-1 Env should facilitate studies of envelope glycoprotein structure and function. The WT cells, which readily induce syncytia with CD4+ cells, provide a convenient system for assessing potential fusion inhibitors and for studying the fusion mechanism of the HIV Env glycoprotein.

Transporter-independent processing of HIV-1 envelope protein for recognition by CD8+ T cells

CD8+ cytolytic T lymphocytes (CTL) identify virally infected cells by recognizing processed viral antigen in association with class I major histocompatibility complex (MHC) molecules on infected cells. Processing begins in the cytosol with the generation of peptides, possibly by a protease complex with MHC-encoded subunits, known as the proteasome. Transport of the resulting cytosolic peptides into the endoplasmic reticulum for association with class I molecules is essential and probably involves a heterodimer of the MHC-encoded proteins, Tap-1 and Tap-2. The site of processing of viral envelope proteins is uncertain. These proteins are not present in the cytosol because of cotranslational translocation into the endoplasmic reticulum. We show here that the HIV-1 envelope (env) protein is processed in infected cells by a novel Tap-1/Tap-2-independent pathway that seems to be localized to the endoplasmic reticulum.

Legitimate and illegitimate cleavage of human immunodeficiency virus glycoproteins by furin

Coexpression of human immunodeficiency virus type 1 glycoproteins with the subtilisin-like protease furin leads to processing of gp160 and gp140, a truncated form of gp160, to gp120. In addition, we show that gp120 itself is further cleaved by furin at sites near the primary cleavage site and within the V3 loop.

Mapping the determinants of human immunodeficiency virus 2 for infectivity, replication efficiency, and cytopathicity

Human immunodeficiency virus 2 (HIV-2) ISY and the newly derived HIV-2KR are infectious molecular clones that yield viruses differing markedly in their abilities to infect and/or induce syncytia in various T- and monocytoid-cell lines. Chimeric viruses were constructed from these two viral genomes to localize the genetic determinants of some of these properties. Envelope sequences, particularly those spanning the CD4 binding site, appear to be critical for the ability of HIV-2KR to infect MOLT-4 clone 8 and SupT1 cells and to efficiently infect the H9 cell line. On the other hand, multiple determinants may contribute to cytopathicity (gp41 and nef) in H9 cells and replication efficiency in monocytic (THP-1) cells.

Analysis of multiple mRNAs from pathogenic equine infectious anemia virus (EIAV) in an acutely infected horse reveals a novel protein, Ttm, derived from the carboxy terminus of the EIAV transmembrane protein

Transcription of pathogenic equine infectious anemia virus (EIAV) in an acutely infected horse was examined by using the polymerase chain reaction and nucleotide sequencing. Four spliced transcripts were identified in liver tissue, in contrast to the multiplicity of alternatively spliced messages reported for in vitro-propagated human immunodeficiency virus, simian immunodeficiency virus, and, to a lesser extent, EIAV. Nucleotide sequence analysis demonstrated that three of these mRNAs encode known viral proteins: the envelope precursor, the product of the S2 open reading frame, and the regulatory proteins Tat and Rev. The fourth transcript encodes a novel Tat-TM fusion protein, Ttm. Ttm is a 27-kDa protein translated from the putative tat CTG initiation codon and containing the carboxy-terminal portion of TM immediately downstream from the membrane-spanning domain. p27ttm is expressed in EIAV-infected canine cells and was recognized by peptide antisera against both Tat and TM. Cells transfected with ttm cDNA also expressed p27ttm, which appeared to be localized to the endoplasmic reticulum or Golgi apparatus by indirect immunofluorescence. The carboxy terminus of lentiviral TM proteins has previously been shown to influence viral infectivity, growth kinetics, and cytopathology, suggesting that Ttm plays an important role in the EIAV life cycle.

Mutations in the cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein impair the incorporation of Env proteins into mature virions

In-frame stop codons were introduced into the coding region of human immunodeficiency virus type 1 (HIV-1) transmembrane protein (gp41). Truncation of 147 amino acids from the carboxyl terminus of gp41 (TM709) significantly decreased the stability and cell surface expression of the viral Env proteins, while truncation of 104 amino acids (TM752) did not. Truncation of 43 or more amino acids from the carboxyl terminus of gp41 generated mutant viruses which were noninfectious in several human CD4+ T lymphoid cell lines and fresh peripheral blood mononuclear cells. Analysis of the noninfectious mutant virions revealed significantly reduced incorporation of the Env proteins compared with the wild-type virions. Comparable amounts of Env proteins were detected on the surfaces of wild-type- and TM752-transfected cells, suggesting that the structures of gp41 required for efficient incorporation of Env proteins were disrupted in mutant TM752. Truncation of the last 12 amino acids (TM844) from the carboxyl terminus of gp41 did not significantly affect the assembly and release of virions or the incorporation of Env proteins into mature virions. However, the TM844 virus had dramatically decreased infectivity compared with the wild-type virus. This suggests that the cytoplasmic domain of gp41 also plays a role in other steps of virus replication.

Analysis of a human immunodeficiency virus type 1 isolate carrying a truncated transmembrane glycoprotein

We have recently reported the isolation of a human immunodeficiency virus type 1 (HIV-1), KB-1gp32 carrying a shorter size (32 kDa) of transmembrane glycoprotein (TMP) from TALL-1 cells persistently infected with KB-1gp41 virus strain (Shimizu et al., 1990a). Endoglycosidase treatments showed that the different size of the TMP between the two strains was due to a truncation of 9 kDa of polypeptide in the KB-1gp32 TMP coding region. Sequence analysis revealed the substitution of a CAG codon to a TAG stop codon just downstream of the putative membrane-spanning domain of the TMP of KB-1gp32. This resulted in a truncation of some 133 amino acids of the cytoplasmic domain of TMP. The data indicate that a premature stop codon in KB-1gp32 has been introduced during adaption of the parental virus to TALL-1 cells. We have constructed two chimeric clones between the env region of a clone pKB-1, derived from KB-1gp32, and an infectious molecular clone pNL-432. We have also constructed a site-directed mutant of pNL-432 carrying a premature stop codon at the same position as the env stop codon of pKB-1. Among the three clones carrying a premature stop codon in env, only one chimeric clone was infectious to TALL-1 but not MT-2 cells. This clone contained the entire tat, rev, vpu, and env genes of pKB-1. The pNL-432 mutant was not infectious. The results suggest that some sequences of pKB-1 might compensate for the truncation of the TMP during replication in TALL-1 cells.

Retained in vitro infectivity and cytopathogenicity of HIV-1 despite truncation of the C-terminal tail of the env gene product

Five in-frame stop mutations in the HIV-1 env gene, which lead to the production of env gene products truncated within the cytoplasmic C-terminal tail, have been generated and their effects on membrane fusion capacity, glycoprotein incorporation into virus particles, infectivity, and cytopathogenicity were analyzed. The resulting truncated glycoproteins were processed normally, were transported to the cell surface, and were able to induce CD4-dependent membrane fusion. The membrane fusion capacity of one of the mutant glycoproteins with a truncation of 144 amino acids was increased to about double of that induced by wild-type glycoprotein. With a single exception, the truncated viral glycoproteins were incorporated into virus particles which were infectious and cytopathic for permissive MT-4 cells. The infection kinetics with the mutated viruses were, however, delayed to varying degrees in comparison to infection with wild-type virus. Nevertheless, in each case, PCR amplification and direct sequencing of viral DNA in the infected cultures confirmed the presence of the mutant and the absence of revertant DNA. The mutant virus encoding a viral glycoprotein with the longest truncation (144 amino acids), in which only 7 cytoplasmic C-terminal amino acids in gp41 remain, resulted in infection kinetics in MT-4 cells which were only marginally delayed in comparison to those induced by wild-type virus. This means that these C-terminal 144 amino acids of gp41 are not necessary for glycoprotein incorporation into virus particles nor do they significantly contribute to the infectivity nor the cytopathogenicity of HIV-1 in MT-4 cells.

Cytoplasmic domain truncation enhances fusion activity by the exterior glycoprotein complex of human immunodeficiency virus type 2 in selected cell types

To investigate the glycoprotein determinants of viral cytopathology, we constructed chimeric env genes between a noncytopathic strain of human immunodeficiency virus type 2 (HIV-2), designated HIV-2/ST, and a highly fusogenic and cytopathic variant derived from this virus. Expression of the resulting chimeric glycoproteins indicated that efficient syncytium formation in the human T-cell line Sup T1 mapped to the C-terminal region of the transmembrane (TM) glycoprotein subunit. In this region, the wild-type and cytopathic ST glycoproteins differed by only four amino acids and by the presence of a premature termination codon in the cytopathic variant. Subsequent site-directed mutagenesis indicated that the cytoplasmic domain truncation was responsible for the enhanced fusion activity. This modification, however, increased the fusion activity of the glycoprotein only in Sup T1 cells (in which the ST variant arose) but not in Molt 4 clone 8 or peripheral blood mononuclear cells. These observations indicate that the length of the cytoplasmic domain of the HIV-2 glycoprotein modulates the fusion activity of the exterior glycoprotein complex in a cell-specific manner. Such adaptability appears to permit the emergence of fusogenic variants during HIV-2 passage in vitro and may also regulate viral growth or cytopathic effects in selected cell types during natural infection in vivo.

Effects of deletions in the cytoplasmic domain on biological functions of human immunodeficiency virus type 1 envelope glycoproteins

The role of the cytoplasmic domain of the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins in virus replication was investigated. Deletion of residues 840 to 856 at the carboxyl terminus of gp41 reduced the efficiency of virus entry during an early step in the virus life cycle between CD4 binding and formation of the DNA provirus without affecting envelope glycoprotein synthesis, processing, or syncytium-forming ability. Deletion of residues amino terminal to residue 846 was associated with decreased stability of envelope glycoproteins made in COS-1 cells, but this phenotype was cell type dependent. The cytoplasmic domain of gp41 was not required for the incorporation of the HIV-1 envelope glycoproteins into virions. These results suggest that the carboxyl terminus of the gp41 cytoplasmic domain plays a role in HIV-1 entry other than receptor binding or membrane fusion. The cytoplasmic domain of gp41 also affects the stability of the envelope glycoprotein in some cell types.

A specific inhibitor of cysteine proteases impairs a Vif-dependent modification of human immunodeficiency virus type 1 Env protein

The Vif protein of human immunodeficiency virus type 1 (HIV-1) regulates viral infectivity. Virions produced in cell culture after transfection by a Vif-negative molecular clone show a dramatic decrease in infectivity for susceptible CD4+ cell lines, although the Vif protein does not appear to be a constituent of the viral particle. The exact mechanism by which Vif affects HIV-1 infectivity is so far unknown. We report the existence of structural homologies between Vif and a family of cysteine proteases and present evidence which suggests that one of the targets of Vif is the Env protein and more precisely the cytoplasmic domain of gp41. Vif was found to modify both the processing and conformation of the Env protein. Ethyl(25, 35)- 3[(5)-3-methyl-1-(3-methylbutylcarbamoyl)]oxirane-2-carboxylate, a specific inhibitor of cysteine proteases, inhibits the effect of Vif, as does the mutation of Cys-114 to Leu in Vif. Furthermore, Cys-114 of Vif produced in Escherichia coli, interacts directly with trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane. These observations suggest that a cysteine protease activity is associated with Vif and that this activity plays a role in Env maturation.

Formation of noninfectious HIV-1 virus particles lacking a full-length envelope protein

Deletions were constructed within a functional human immunodeficiency virus type 1 (HIV-1) proviral clone in order to assess the role of the envelope protein in virus particle formation. A graded exonuclease deletion technique was used to produce 12 clones with deletions of 175-308 nucleotides in the first conserved domain of envelope. This included 9 clones with frameshift deletions and 3 clones with in-frame deletions. Isogenic pairs of env deletion clones were produced with or without an additional deletion in the vif and vpr genes. Upon transfection, all clones produced virus particles, as determined by p24 antigen, reverse transcriptase, and sucrose gradient assays with conditioned media. Virus particles produced from clones with deletions in env or vif and vpr, or both regions, banded on sucrose gradients with a mobility similar to that of virus produced by the parental clone. The p24 gag capsid protein in the particles was resistant to trypsin, but the particles were disrupted by treatment with Triton X-100, suggesting the presence of a surrounding lipid bilayer. Furthermore, electron microscopic studies revealed both mature and immature virus particles derived from COS cells transfected with the env deletion clones. Cocultivation experiments with lymphoid cells and cells transfected with each of the env deletion clones demonstrated that the virus particles were noninfectious.

The cytoplasmic tail of HIV-1 gp160 contains regions that associate with cellular membranes

The HIV-1 envelope glycoprotein gp160 associates with cellular membranes via a discrete transmembrane domain. Unlike other retroviral envelope proteins, however, gp160 also forms a secondary association with the lipid bilayer mediated by one or more regions located in the cytoplasmic tail. We have expressed the full cytoplasmic tail sequence of gp160, as a fusion protein with the HSV-1 glycoprotein D signal sequence, transiently in a human embryonic kidney cell line. Our results show that in the absence of any defined transmembrane domain or stop transfer sequence, the protein corresponding to the cytoplasmic tail of HIV-1 gp160 formed stable interactions with cellular membranes that mediated its export to the cell surface.

Human immunodeficiency virus-like, nonreplicating, gag-env particles assemble in a recombinant vaccinia virus expression system

We report the assembly of human immunodeficiency virus (HIV)-like particles in African green monkey kidney cells coinfected with two recombinant vaccinia viruses, one carrying the HIV-1 gag and protease genes and the other the env gene. Biochemical analysis of particles sedimented from culture supernatants of doubly infected cells revealed that they were composed of gag proteins, primarily p24, as well as the env proteins gp120 and gp41. Thin-section immunoelectron microscopy showed that these particles were 100 to 120 nm in diameter, were characterized by the presence of cylindrical core structures, and displayed the mature gp120-gp41 complexes on their surfaces. Furthermore, thin-section immunoelectron microscopy analysis of infected cells showed that particle assembly and budding occurred at the plasma membrane. Nucleic acid hybridization suggested that the particles packaged only the gag mRNA but not the env mRNA. Therefore, the system we present is well suited for studies of HIV virion maturation. In addition, the HIV-like particles provide a novel and attractive approach for vaccine development.